This invention relates to narrowband phase modulation systems for voice communication.
Narrowband frequency modulation (FM) systems will, by definition, exhibit significant postdetection noise and although the postdetection signal-to-noise ratio may be increased by applying conventional preemphasis and deemphasis filtering, the best that can be done is to achieve substantially flat gaussian noise in consequence of which the high frequency components of the voice signal are masked by the demodulated noise. Moreover, in any FM system there is a definite thresholding limitation which exists even when the received signal-to-noise ratio is sufficiently large as to provide an otherwise usable message signal. That is to say, although for some low value of signal-to-noise ratio the message in any event will be mutilated beyond all recognition by the noise, significant mutilation due to thresholding may begin to occur at much larger ratios. Under this latter condition, the signal and noise phasors are of nearly equal length so that if they also are of nearly opposite phase, their resultant is quite small and a small change then in noise phase yields a large phase deviation of the resultant. A sudden stepwise phase deviation is equivalent to an impulsive frequency shift and in consequence such noise-induced phase deviations produce sharp pulses or spikes at the discriminator output, recognized aurally as a click or cracking sound. This phenomenon of thresholding is well known and is characterized by the fact, apparent from the above, that it occurs randomly when the signal-to-noise ratio is otherwise useful.